In recent years, as industrial equipment comes to have higher performance, a power module to be mounted thereon is being made to have a higher output. With the above, a semiconductor element is being made to have a higher output. An operation guaranteed temperature of the semiconductor element is 125° C. to 150° C., but there is a possibility that the operation guaranteed temperature is raised to 175° C. or more in the future.
With rise of the operation guaranteed temperature of the semiconductor element, a high TCT characteristic is demanded of a ceramic metal circuit board. TCT stands for a thermal cycle test. The TCT is a method of measuring durability of the ceramic metal circuit board by carrying out a process of changing a temperature in order of low temperature→room temperature→high temperature→room temperature as one cycle.
A conventional ceramic metal circuit substrate has a ceramic substrate and a metal plate. The metal plate is bonded to the ceramic substrate via a bonding layer formed by using a brazing material. The bonding layer has a protruding portion which extends onto the ceramic substrate in a manner to protrude from between the ceramic substrate and the metal plate. The above-described ceramic metal circuit substrate has a high durability to the TCT of 5000 cycles. By reducing a gap of the protruding portion, the TCT characteristic can be improved. However, when the operation guaranteed temperature is 175° C. or more, improvement of the TCT characteristic by merely removing the gap of the protruding portion is limited.
A power density indicating performance of a power module is obtained by the following formula.power density=(rated current×rated voltage×number of semiconductor elements mounted on module/volume of module
The power density can be made large by increasing the number of the semiconductor elements mounted on the module or decreasing the volume of the module, for example. In order to improve these two parameters, it is required to mount a plurality of semiconductor elements on a ceramic metal circuit board. In order to realize a ceramic metal circuit board on which a plurality of semiconductor elements can be mounted and which is small in volume, it is preferable to narrow an arrangement interval between a plurality of metal plates. Also in a case of narrowing the arrangement interval between the metal plates, improvement of a TCT characteristic at a high temperature of 175° C. or more is required of the ceramic metal circuit board.